When a semiconductor is exposed to light, electrons having a strong reducing action and holes having a strong oxidizing action are generated. Consequently, a molecular species that contacts the semiconductor is decomposed by oxidation-reduction action. This action of a semiconductor is referred to as photocatalytic action, and ever since the discovery of photodecomposition of water using a semiconductor photoelectrode (the so-called Honda-Fujishima effect), extensive research has been conducted on this action as an effective means of converting light energy to chemical energy. In addition, attempts have been made to utilize this principle to apply semiconductors to, for example, 1) oxidation of organic compounds, 2) organic synthesis such as hydrogenation of unsaturated compounds, 3) removal and decomposition of hazardous chemical substances in waste liquids or exhaust gas, and 4) sterilization or decontamination.
Known examples of semiconductor photocatalytic materials include titanium dioxide (titania) as well as vanadium pentoxide, zinc oxide, tungsten oxide, copper oxide, iron oxide, strontium titanate, barium titanate, sodium titanate, cadmium sulfide, zirconium dioxide and iron oxide. Moreover, co-catalysts obtained by loading metals such as platinum, palladium, rhodium and ruthenium on these semiconductors are also known to be effective as photocatalysts.
Semiconductor powders having a particle diameter on the micron level have frequently been used in conventional photocatalyst research. The conversion of these semiconductor powders into films is essential for the practical application of photocatalysts. Consequently, they are known to be used by fixing to materials such as resins and glass or used in the form of thin films. However, the amount of the catalyst itself is insufficient, and the effect is unsatisfactory. In addition, although the surface area of the catalyst layer may be increased to increase the amount of catalyst, design limitations are normally encountered when this is attempted.
On the other hand, since an electrode output can be obtained from semiconductor materials like those described above when light is radiated onto an n-type semiconductor, they are also used for the electrode materials and so forth of wet photovoltaic cells utilizing photo-sensitive electrolysis phenomena. In recent years, there has been considerable activity in the development of dye-sensitized solar cells in particular. The primary structure of the semiconductor electrode serving as the working electrode consists of a dye sensitizer adsorbed onto a semiconductor porous film. Examples of materials used for these semiconductors include titanium dioxide (titania), tin oxide, zinc oxide and niobium oxide, while ruthenium complexes are used for the sensitizing dye. Although these dye-sensitized solar cells have a simpler structure and are less expensive than conventional silicon solar cells, improvement of conversion efficiency is the issue of greatest importance in terms of achieving practical application.
In a photocatalyst or photoelectrode, the use of a porous or fine particulate oxide semiconductor material has been studied so as to realize a structure having low density and large specific surface area for the oxide semiconductor in order to obtain greater optical activity with less volume. For example, a method has been disclosed for obtaining a titanium oxide porous thin film photocatalyst having pores of a uniform pore diameter of 1 nm to 2 μm by coating a titania sol onto a substrate followed by heating and baking (see, for example, Patent Document 1). In addition, the structure of a metal oxide porous body and production process thereof has been disclosed in which the pore size frequency distribution of fine pores has a plurality of peak values (see, for example, Patent Document 2). Studies have also been conducted on increasing specific surface area by using finer particles. For example, a method has been disclosed for obtaining an oxide semiconductor electrode having a porous oxide semiconductor layer containing hollow particles comprising a metal oxide having an average particle diameter of 200 nm to 10 μm (see, for example, Patent Document 3).
Patent Document 1: Japanese Patent No. 2636158 (pp. 1-3)
Patent Document 2: Japanese Patent No. 3309785 (pp. 1-5)
Patent Document 3: Japanese Published Patent Application No. 2001-76772 (pp. 1-6, FIGS. 1-4)
Examples other documents relating to the present invention include: Sasaki, T. et al.: “Preparation of Metal Oxide Nanoparticles by Laser Ablation”, Laser Research, Vol. 28, No. 6, June 2000,
Japanese Published Patent Application No. 2003-142171 (and particularly paragraphs [0107] to [0110]).
The above-mentioned document discloses the obtaining of a semiconductor electrode for a solar cell by coating a slurry containing two types of titanium oxide particles having different particle diameters onto a glass substrate followed by drying.
Japanese Published Patent Application No. 2000-106222.
This publication discloses a semiconductor electrode for a solar cell having two types of titanium oxide particles of having different particle diameters.
Japanese Published Patent Application No. 2002-134435.
International Publication No. WO 00/30747